Hi! Where can I find an example of sending an external message to smart contract? I'm wondering how I can call the recive_external function via the web interface.

Why are you using external? Are you developing your own wallet contract?

https://answers.ton.org/question/1485659652266921984/what-s-the-difference-between-internal-and-external-messages-in-ton-smart-contracts
in almost all scenarios internal message is the right one to use